The present invention relates generally to a hologram color filter and its fabrication method, and more particularly to a hologram color filter for liquid crystal display devices which is much more reduced in terms of dependence of diffraction efficiency on wavelength and so is well corrected for a color balance among three colors R, G and B, and its fabrication method.
Moreover, the present invention relates generally to an alignment mark and method, and more specifically to an alignment mark for hologram color filters and a method of aligning a hologram color filter and a back matrix.
Applicant has already filed Japanese Patent Application No. 5-12170, etc., to propose a color filter for color liquid crystal display devices, which enables the respective wavelength components of backlight to be more efficiently incident on liquid crystal cells without wasteful absorption as compared with a conventional wavelength absorption type of color filter, whereby the efficiency of utilization of backlight can be greatly improved. This color filter is generally broken down into two types, one of which makes use of an array of an eccentric Fresnel zone plate form of microholograms. Another type utilizes an array of micro-lenses superposed on a hologram or diffraction grating having parallel and uniform interference fringes thereon. A brief account will now be given of these hologram color filters.
A liquid crystal display device making use of the first type of hologram color filter is explained with reference to FIG. 11 that is a sectional schematic thereof. As shown, a hologram array 5 forming this hologram color filter is spaced away from the side of a liquid crystal display element 6 on which backlight 3 is to be incident, said element 6 being regularly divided into liquid crystal cells 6xe2x80x2 (pixels). On the back side of the liquid crystal display element 6 and between the liquid crystal cells 6xe2x80x2 there are located black matrices 4. Although not illustrated, polarizing plates are arranged on both sides of the liquid crystal display element 6. As is the case with a conventional color liquid crystal display device, between the black matrices 4 there may additionally be located an absorption type of color filters which transmit light rays of colors corresponding to red, green and blue pixels.
The hologram array 5 comprises microholograms 5xe2x80x2 which are arranged in an array form at the same pitch as that of red, green and blue pixels, corresponding to the period of repetition of red, green and blue pixels, i.e., sets of liquid crystal cells 6xe2x80x2, each including three adjoining liquid crystal cells 6xe2x80x2 of the liquid crystal display element 6 as viewed in a longitudinal direction thereof. One microhologram 5xe2x80x2 is located in line with each set of three adjoining liquid crystal cells 6xe2x80x2 of the liquid crystal display element 6 as viewed in the longitudinal direction thereof. The microholograms 5xe2x80x2 are then arranged in a Fresnel zone plate form such that a green component ray of the backlight 3 incident on the hologram array 5 at an angle xcex8 with respect to its normal line is converged at a middle liquid crystal cell G of the three red, green and blue pixels corresponding to each microhologram 5xe2x80x2. Each or the microhologram 5xe2x80x2 in this case is constructed from a relief, phase, amplitude or other transmission type of hologram which has little, if any, dependence of diffraction efficient on wavelength. The wording xe2x80x9clittle, if any, dependence of diffraction efficiency on wavelengthxe2x80x9d used herein is understood to refer specifically to a hologram of the type which diffracts all wavelengths by one diffraction grating, much unlike a Lippmann type hologram which diffracts a particular wavelength alone but does not substantially permit other wavelengths to be transmitted therethrough. The diffraction grating having little dependence of diffraction efficiency on wavelength diffracts different wavelengths at different angles of diffraction.
In such an arrangement, consider the incidence of the white backlight 3 from the side of the hologram array 5, which does not face the liquid crystal display element 6 at the angle xcex8 with respect to its normal line. The angle of diffraction of the light by the microholograms 5xe2x80x2 varies depending on wavelength, so that convergence positions for wavelengths are dispersed in a direction substantially parallel with the surface of the hologram array 5. If the hologram array 5 is constructed and arranged such that the red wavelength component is diffractively converged at a red-representing liquid crystal cell R; the green wavelength component at a green-representing liquid crystal cell G; and the blue wavelength component at a blue-representing liquid crystal cell B, the color components transmit the corresponding liquid crystal cells without undergoing little or no attenuation through the black matrices 4, so that color displays can be presented depending on the state of the liquid crystal cells 6xe2x80x2 at the corresponding positions.
By using the hologram array 5 as a color filter in this way the wavelength components of backlight used with a conventional color filter are allowed to be incident on the liquid crystal cells"" without extravagant absorption, so that the efficiency of utilization thereof can be greatly improved.
A liquid crystal display device with the second type of hologram color filter built in it is then explained with reference to FIG. 12 that is a sectional schematic thereof. As illustrated, the second type of hologram color filter generally shown at 10 comprises a hologram 7 and a converging microlens array 8. Microlenses 8xe2x80x2 forming part of the microlens array 8 are arranged in an array form at the same pitch as that of red, green and blue pixels, corresponding to the period of repetition of red, green and blue pixels, i.e., sets of liquid crystal cells 6xe2x80x2, each including three adjoining liquid crystal cells 6xe2x80x2 of a liquid crystal display element 6 as viewed in a longitudinal direction thereof. The hologram 7 is made up of a relief, phase, amplitude or other transmission type of hologram which has thereon parallel and uniform interference fringes that act as a diffraction grating, and has little or no dependence of diffraction efficiency on wavelength. On the back surface of the liquid crystal display element 6 and between the liquid crystal cells 6xe2x80x2 there are located black matrices 4. Although not illustrated, polarizing plates are arranged on both sides of the liquid crystal display element 6. As is the case with a conventional color liquid crystal display device, between the black matrices 4 there may additionally be located an absorption type of color filters which transmit light rays of colors corresponding to red, green and blue pixels.
In such an arrangement, consider the incidence of the white backlight 3 from the side of the hologram 7 that is not opposite to the liquid crystal display element 6 at an angle xcex8 with respect to its normal line. The incident light is diffracted at different angles depending on wavelength, and then emerges dispersively from the hologram 7. The dispersed light is in turn separated for each wavelength by the microlenses 8xe2x80x2 located on an incident or emergent side of the hologram 7, so that it is converged at focal surfaces thereof. If the color filter 10 is constructed and arranged such that the red wavelength component is diffractively converged at a red-representing liquid crystal cell R; the green wavelength component at a green-representing liquid crystal cell G; and the blue wavelength component at a blue-representing liquid crystal cell B, the color components transmit the corresponding liquid crystal cells 6xe2x80x2 without undergoing little or no attenuation through the black matrices 4, so that color displays can be presented depending on the state of the liquid crystal cells 6xe2x80x2 at the corresponding positions.
In such a layout, a transmission type of non-converging hologram made up of uniform interference fringes and having little, if any, dependence of diffraction efficiency on wavelength can be used as the hologram 7. Thus, this layout has the advantages of dispensing with any alignment of the hologram 7 with the microlenses 8xe2x80x2 forming part of the microlens array 8, and of being easy to make and align because the pitch of the microlens array 8 is three times as large as that of a conventional layout where one microlens is used for each liquid crystal cell 6xe2x80x2.
A modification of FIG. 12 is illustrated in FIG. 13, wherein a microlens array 8 and a liquid crystal display element 6 are located as shown in FIG. 5 with the exception that a hologram 7 made up of parallel and uniform interference fringes that act as a diffraction grating is separate from the microlens array 8 and positioned in backlight 3 almost perpendicularly to the direction of its propagation. In this layout, too, the wavelength components of the backlight are permitted to be incident on liquid crystal cells 6xe2x80x2 without extravagant absorption, thus making it possible to achieve a color filter greatly improved in terms of the efficiency of utilization of the backlight.
Such a hologram color filter 5 as mentioned above is fabricated by making a computer generated hologram array and replicating it. More exactly, hologram interference fringes to be defined by the microholograms 5xe2x80x2 are computed by a computer, written by electron beams onto an electron beam resist coated on a glass substrate with a chromium film, for instance, being formed thereon, and developed to form a chromium pattern for a relief type of computer generated hologram (CGH) array. Then, the glass substrate is subjected to ion etching using the chromium pattern as a mask to make an original CGH array. Subsequently, while a hologram photosensitive material is superposed on a relief surface of the thus prepared CGH array either in close contact relation to each other or with some gap between them, laser light is directed through the CGH array to the photosensitive material at an angle xcex8 corresponding to the backlight 3 shown in FIG. 11 to cause interference of converging diffracted light and rectilinearly propagating transmitted light produced by CGHs of the CGH array to occur in the hologram photosensitive material, so that the CGH array can be replicated. This replicated hologram is used as the hologram array 5 shown in FIG. 8. Alternatively, a replica of such a replicated hologram may be used as the hologram array 5.
For the purpose of wavelength dispersion, the aforesaid hologram color filter already put forward by the applicant makes use of a hologram having little, if any, dependence of diffraction efficiency on wavelength. When used in practical applications, however, the hologram color filter tends to suffer from diffraction efficiency variations by reason of the diffraction theory per se and because the hologram used has some thickness. Especially for a hologram color filter designed to present liquid crystal displays in the three colors R, G and B, it is desired that a peak of a ridgeline form of diffraction efficiency be located at a region of center wavelength G, thereby placing the three colors in a well-balanced state; diffraction efficiencies of the wavelength regions R and B become lower than that of the wavelength region G. As a consequence, the three colors R, G and B vary- in intensity, resulting in ill-balanced color displays.
To build such a hologram color filter 5 as mentioned above in a liquid crystal display device, it is required that the hologram array 5 be brought into precise alignment with the black matrix 4 located on the back side of the liquid crystal display element 6.
However, the position of the black matrix 4 can be identified in the form of a contrast image, but it is impossible to identify the position of the hologram array 5 in the form of a contrast image by means of ordinary observing techniques, because the hologram array 5 is usually constructed from a phase type of holograms. In addition, as can be seen from the layout shown in FIG. 11, it is impossible to keep the hologram array 5 and black matrix 4 in precise alignment by means of alignment techniques designed to observe the same plane, because they are spaced away from each other at an interval corresponding approximately to the focal length of the microhologram 5xe2x80x2.
In view of the aforesaid problems associated with prior art hologram color filters, an object of the present invention is to provide a hologram color filter having a remarkably decreased dependence of diffraction efficiency on wavelength and well corrected for a color balance among the three colors R, G and B, and a fabrication method thereof.
Another object of the present invention is to provide an alignment mark used with a phase type of holograms for hologram color filters or the like, and an alignment method using such an alignment mark, especially an alignment mark best suited for bringing a hologram color filter in alignment with pixels of a liquid crystal display element and an alignment method.
To accomplish the aforesaid first object, the present invention provides a hologram color filter comprising an array of converging element holograms, each of which enables white light incident at a given angle with respect to a normal line of a hologram recorded surface thereof to be spectroscopically separated by wavelength dispersion in a direction substantially along the hologram recorded surface, characterized in that said converging element holograms have each a plurality of hologram pieces superposed on each other or multi-recorded therein, which, with respect to said white light incident at a given angle, have substantially identical spatial wavelength distributions of wavelength dispersion and different peak wavelengths of diffraction efficiency.
In this case, it is desired that the superposed or multi-recorded hologram pieces have substantially identical convergence distances at peak wavelengths of diffraction efficiency thereof.
According to the present invention, there is also provided a hologram color filter having a hologram comprising parallel and uniform interference fringes and an array of converging element lenses located on an incident or emergent side thereof, said converging element lenses being each cooperative with said hologram to enable white light incident at a given angle on a hologram recorded surface thereof to be spectroscopically separated by wavelength dispersion in a direction substantially along the hologram recorded surface, characterized in that said hologram comprising parallel and uniform interference fringes has a plurality of hologram pieces superposed on each other or multi-recorded therein, which, with respect to said white light incident at a given angle, have substantially identical spatial wavelength distributions of wavelength dispersion and different peak wavelengths of diffraction efficiency.
In these hologram color filters, it is desired that the spatial wavelength distributions of wavelength dispersion of said superposed or multi-recorded hologram pieces be shifted to each other by an angle of at least 1xc2x0 between principal rays of central wavelength.
These hologram color filters are preferably used in a color liquid crystal display device having black matrices located between pixels.
To fabricate such hologram color filers, the present invention provides a method of fabricating a hologram color filter comprising an array of converging element holograms, each of which enables white light incident at a given angle with respect to a normal line of a hologram recorded surface thereof to be spectroscopically separated by wavelength dispersion in a direction substantially along the hologram recorded surface, wherein said converging element holograms have each a plurality of hologram pieces superposed on each other or multi-recorded therein, which, with respect to said white light incident at a given angle, have substantially identical spatial wavelength distributions of wavelength dispersion and different peak wavelengths of diffraction efficiency, characterized in that reference light having the same wavelength as one of said peak wavelengths and incident at the same angle of incidence as that of white light for reconstruction and object light converging toward a point at which light of that wavelength is to converge during reconstruction or object light propagating in a direction in which light of that wavelength is to be diffracted during reconstruction are permitted to be concurrently incident on a hologram photosensitive material to record a first hologram piece therein, and simultaneously with or subsequently to this, a second hologram piece is recorded in the hologram photosensitive material using light of the same wavelength as another peak wavelength, similar recording operation being repeated plural times.
In this case, the object light at each peak wavelength is generated in the form of diffracted light obtained by permitting reconstruction illumination light to be incident at the same angle of incidence as white light for reconstruction on an identical computer generated hologram and diffracting said reconstruction illumination light by said computer generated hologram, and the reference light at each peak wavelength is generated in the form of rectilinearly propagating diffracted light of said reconstruction illumination light by said computer generated hologram. Alternatively, an array of holograms each having a plurality of the fabricated hologram pieces superposed on each other or multi-recorded therein is used in place of said computer generated hologram to generate object light and reference light in similar manners, whereby similar recording operation is repeated plural times.
One modification of such a method of fabricating a hologram color filter is characterized in that reference light having a given wavelength and incident at a first angle different from an angle of incidence of white light for reconstruction and object light converging toward a point at which light of that wavelength is to converge during reconstruction are permitted to be concurrently incident on a hologram photosensitive material to record a first hologram piece therein, and simultaneously with or subsequently to this, reference light having said given wavelength and incident at a second angle different from the angle of incidence of white light for reconstruction and said first angle and object light converging toward a point at which light of that wavelength is to converge during reconstruction are permitted to be concurrently incident on the hologram photosensitive material to record a second hologram piece therein, similar recording operation being repeated plural times.
Another modification of such a method of fabricating a hologram color filter is characterized in that reference light having a first wavelength and incident at a first angle different from an angle of incidence of white light for reconstruction and object light converging toward a point at which light of that wavelength is to converge during reconstruction are permitted to be concurrently incident on a hologram photosensitive material to record a first hologram piece therein, and simultaneously with or subsequently to this, reference light having a second wavelength and incident at a second angle different from the angle of incidence of white light for reconstruction and said first angle and object light converging toward a point at which light of that wavelength is to converge during reconstruction are permitted to be concurrently incident on the hologram photosensitive material to record a second hologram piece therein, similar recording operation being repeated plural times.
To accomplish the aforesaid second object, the present invention provides an alignment mark provided on the same substrate as that for a hologram or diffraction grating, characterized by comprising interference fringes or a diffraction grating.
In this case, the hologram or diffraction grating comprises a hologram color filter comprising an array of periodically arranged converging element holograms, each of which enables white light incident at an angle with respect to a normal line of a hologram recorded surface thereof to be spectroscopically separated by wavelength dispersion in a direction along the hologram recorded surface.
Also, the interference fringes or diffraction grating of said alignment mark comprise phase interference fringes or a phase diffraction grating.
The hologram or diffraction grating, too, comprises phase interference fringes or a phase diffraction grating similar to said alignment mark.
It is here to be noted that either a converging phase hologram or a phase diffraction grating having a constant pitch may be used as the alignment mark.
The present invention provides another alignment method of bringing one substrate which is identical with that for a hologram or diffraction grating and is provided thereon with an alignment mark comprising a converging hologram in alignment with another substrate having an opposing alignment mark thereon, characterized in that the alignment mark on said one substrate is illuminated from a given direction to take an image of a convergence point of diffracted light while an image of the opposing alignment mark on said another substrate is taken, and the thus taken images are both displayed on an identical screen to regulate the relative positions of said both substrates.
The present invention provides still another alignment method of bringing one substrate which is identical with that for a hologram or diffraction grating and is provided thereon with an alignment mark comprising a converging hologram in alignment with another substrate having an opposing alignment mark thereon, characterized in that the alignment mark on said one substrate is illuminated from a given direction to form a convergence point of diffracted light in the vicinity of the opposing alignment mark on said another substrate while an image of the vicinity of the opposing alignment mark on said another substrate is taken, and the thus taken images are both displayed on an identical screen to regulate the relative positions of said both substrates.
The present invention provides a further alignment method of bringing one substrate which is identical with that for a hologram or diffraction grating and is provided thereon with an alignment mark comprising a diffraction grating having a constant pitch in alignment with another substrate having an opposing alignment mark thereon, characterized in that the alignment mark on said one substrate is illuminated from a given direction to take an image of the vicinity of said alignment mark by a rectilinearly propagating component or a diffracted component while an image of the opposing alignment mark on said another substrate is taken, and the thus taken images are both displayed on an identical screen to regulate the relative positions of said both substrates.
The present invention provides a still further alignment method of using an alignment mark comprising interference fringes or a diffraction grating provided on a substrate identical with that for a hologram or diffraction grating in given relation to said hologram or diffraction grating to detect a position of said substrate, characterized in that said alignment mark is illuminated from a given direction to take an image of a convergence point of diffracted light or take an image of the vicinity of said alignment mark by a rectilinearly propagating component or a diffracted component, thereby obtaining a contrast image thereof, on the basis of which said substrate can be subjected to given machining.
In the hologram color filter(s) according to the present invention and its fabrication method(s), the converging element holograms forming the hologram color filter or the hologram comprising parallel and uniform interference fringes are each constructed from two hologram pieces superposed on each other or multi-recorded therein, which, with respect to white light incident at a given angle, have substantially identical spatial wavelength distributions of wavelength dispersion and different peak wavelengths of diffraction efficiency. The composite diffraction efficiency distribution given by the two hologram pieces can be made wider and gentler than would be possible with a single hologram, so that a satisfactory color balance is achievable. It is also possible to place the color balance under free control, when it becomes unfavorable due to the geometry of an opening pattern between black matrices, a spectral distribution of a light source, etc., so that the color balance can be corrected with simple arrangements to thereby achieve the optimum color reproduction.
Referring to the alignment mark(s) and alignment method(s) according to the present invention, the alignment mark can be fabricated simultaneously with the fabrication of a main hologram or diffraction grating, because the alignment mark, which comprises interference fringes or a diffraction grating, is designed to be provided on the same substrate as that for a hologram or diffraction grating.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises in the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.